Topex was JPL's follow-on to the Seasat-A mission of 1978. Unlike Seasat, Topex had just one scientific instrument, a radar altimeter designed to measure sea surface height. Surface height is directly related to temperature, tides and currents, all of which are important to oceanographers and meteorologists. But in the poor fiscal atmosphere of the early 1980s, NASA could not afford the project. Similarly, the French space agency, Centre National d'Etudes Spatiales, had a nearly identical "Poseidon" mission in its plans that it could not afford. So, NASA associate administrator Burt Edelson arranged a merger with his French counterpart, Jean-Louis Fellouf, with NASA providing the satellite, France providing an Ariane launch vehicle, and both providing radar altimeters.

Arianespace launched Topex/Poseidon on Oct. 10, 1992, from its facility in Korou, French Guyana. Over its life, the mission proved the existence of deep-ocean waves previously known only from theory, watched the complete evolution of the largest El Nino in the 20th century during 1997 and 1998, showed seasonal cycles in the world ocean, and measured the slow thermal expansion of the oceans as they warm.

Far exceeding its expected five-year life, Topex/Poseidon was finally shut down in January 2006. The mission had been so valuable to Earth science, it had already been replaced by the Jason-1 mission (also a joint U.S./France effort). The two spacecraft shared the same orbit for several years, while scientists compared their data.